It is known in the art that the "ene reaction" of olefins with alpha-, beta-unsaturated carboxylic acid esters for the production of unsaturated carboxylic acid esters can be promoted by a wide variety of materials. Particular interest has, however, been shown in the use of halide catalysts. For instance, U.S. Pat. No. 3,783,136 and German Offenlegungsschrift 2063515 both describe the use of AlCl.sub.3 and AlBr.sub.3 as catalysts for such reactions. U.S. Pat. No. 4,506,095 describes the reaction of linear alpha-olefins with alkyl acrylates catalyzed by an organometallic catalyst of the formula R.sub.n -Al-X.sub.3-n, wherein R is an organic radical containing between about 1 and 12 carbon atoms, n is the integer 1 or 2, and X is chlorine or bromine. The publication by B. R. Snider in J. Org. Chem., vol. 39, no. 2 (1974), p. 255, refers generally to Lewis acid catalysts for ene reactions, and particularly illustrates the use of aluminum chloride and zinc bromide. U.S. Pat. No. 2,093,695 discloses preparation of carboxylic acid esters by reaction of acyloxy compounds with olefinic hydrocarbons catalyzed by the halogens and various halides of calcium, boron, cadmium, zinc, calcium and potassium. Akermark et al (J. Org. Chem., vol. 43, no. 22 (1978), p. 4387) have reported that the eutectic mixture of AlCl.sub.3, NaCl, and KCl is a superior ene reaction catalyst. U.S. Pat. No. 3,892,788 teaches a ligand-stabilized Pt(II) dihalide complex combined with a Group IVb metal halide as a catalyst for such reactions. U.S. Pats. No. 4,009,203 and U.S. Pat. No. 3,534,087, German Offenlegungsschrift 3149979 and World Pat. No. 8100846 describe related reactions of acids and olefins catalyzed by an acyloxy-stannic trihalide or a perfluorosulfonic acid resin or a crystalline metal silicate or an aluminum silicate containing a Group VIII metal compound and a polyvalent metal halide. U.S. Pat. No. 4,822,911 describes ene reactions catalyzed by gallium chloride, indium chloride and tantallum pentachloride.
It is further recognized that such reactions result in a crude ene reaction product mixture which contains dissolved halide catalyst residues, including the catalyst and/or its degradation products.
The present invention is particularly directed to an improved method for recovering the higher alkyl acrylic acid ester products of the reaction of alkyl acrylic acid esters with olefins having carbon numbers of at least about 10. This improved method includes steps for the extraction of the resulting product mixture with an aqueous acid solution and the distillation of the extracted mixture to remove unreacted olefin starting material.
Although it is known generally known in the art that extraction and distillation steps may be applied in the recovery of the higher ester products of ene reactions, it has now been found that the methods taught in the art cannot be successfully applied to the recovery of high quality ester products derived from C.sub.10 and higher olefins. U.S. Pat. No. 3,783,136, in particular, describes in its Example 1 the recovery of the ester product of the reaction of methyl acrylate and 1-hexene catalyzed by aluminum chloride. That recovery procedure includes washing the product mixture with water, drying with anhydrous sodium sulfate, and distilling to remove unreacted 1-hexene and methyl acrylate. When similar procedures are applied to the treatment of the crude product of the aluminum chloride catalyzed reaction of methyl acrylate with C.sub.10+ olefins, the water wash step results in the formation of a difficult to separate emulsion between the water and organic liquid phases and, in turn, in an organic phase which retains significant amounts of catalyst residues. Under the conditions of elevated temperature required for the subsequent distillation step, the catalyst residues remaining in that washed product then promote reactions (e.g., oligomerization of unreacted olefins and reactions which degrade ester starting material and product) that result in a product of lower overall quality. Both the greater content of catalyst residues and the increased temperature required for distillation of C.sub.10+ olefin (rather than C.sub.6 olefins, as exemplified in U.S. Pat. No. 3,783,136) contribute to a loss of ester product quality.